In recent years, the fluorescent lamp, which requires less energy than the incandescent lamp to produce the same amount of light, has enjoyed increasing popularity. In many modern offices, fluorescent lamps are used to the complete exclusion of incandescent lamps. However, the energy efficient fluorescent lamp has not replaced incandescent lamps to the same extent in other applications.
Since the advent of electrical lighting, people have desired to vary the brightness of lamps, so that a single lamp or group of lamps can provide varying light levels appropriate for a variety of activities. Dimming circuits for incandescent lamps have been well-known for many years, but dimming circuits for fluorescent lamps are more difficult to construct; previous efforts at producing effective fluorescent lamp dimmers have not been entirely successful. This is especially true when dealing with reliable lamp starting and maintaining optimum lamp life.
The present inventor's copending U.S. patent applications, now issued as U.S. Pat. Nos. 4,933,605 and 4,864,482, disclose a solid-state resonant inverter fluorescent ballast circuit which overcomes many of the problems associated with dimming fluorescent lamps. However, to achieve the highest functionality possible, a fluorescent dimming circuit should compensate for several inherent disadvantages of the fluorescent lamp relative to the incandescent lamp. First, fluorescent lamps must be "started" at full intensity. Generally, these lamps will not "start" at all at reduced intensity. In any case, low intensity startups will reduce their lifespan. Second, fluorescent lamps glow as a result of continuous excitation. The level of excitation can be reduced after the initial startup, but even a momentary interruption in this reduced excitation will put the lamp out, so that it must be restarted with greater excitation. Finally, excitation of fluorescent lamps requires storage of potentially dangerous charges, so that it becomes important that the controls for the lamps be isolated from the excitation circuitry.
An efficient, reliable, and useful fluorescent dimming system could be produced by providing the inventor's solid-state, resonant inverter dimming ballast for fluorescent lamps with a simple, reliable solid-state control circuit that compensates automatically for the disadvantages of the fluorescent lamp as described above.